i am dropping this here as i intend to read it later and on the interwebs there are many solution that do not work but this one worked:
why do i need this: if you are playing only one box on a an amp with 2 channels for example you want to have both channels mixed into at least one channel (L or R) or even better to have both channels mixed into both channels (L and R).
this is an non persistent approach that works for my setup i might add comments later or you if you want:
pactl load-module module-remap-sink sink_name=combined-mono master=0 channels=2 master_channel_map=mono,mono channel_map=mono,mono
pactl set-default-sink combined-mono